package: usbgadget: Fix subpackage dependencies

package: usbgadget: Fix subpackage dependencies

Currently if a target wants to use acm or ncm gadget config,
they need to select both usbgadget and the config package
such as usbgadget-acm.

It's better if the target just select usbgadget-acm and get
all dependencies satisfied.

Adding a dependency using +usbgadget fixes this.

Link: http://patchwork.ozlabs.org/project/openwrt/patch/[email protected]/
Reviewed-by: Chuanhong Guo <[email protected]>
Signed-off-by: Linus Walleij <[email protected]>